Electronic governing devices of the analog type have been available for decades. These typically take an a.c. speed signal from an engine-driven tachometer-generator, rectify and filter it to arrive at a d.c. level representative of instantaneous speed, and compare this with a d.c. reference to fine a speed error signal which is then used to correct any speed error. Such circuits are subject to the usual problems with any voltage sensitive analog control in that they are subject to error from temperature and aging effects on the components, from supply voltage variations, etc. so that, for many applications, their accuracy is not what might be desired.
For a gas turbine control system there are other engine operating variables used to determine fuel flow such as compressor discharge pressure, ambient temperature, and turbine temperature. Since in the control system these various analog signals are effectively multiplied together, errors tend to become cumulative. Currently it is desired to provide digital control systems which are much less sensitive to the above sources of error. It therefore becomes useful and basically necessary to provide a speed sensing circuit which will provide an accurate and reliable digital signal representing instantaneous engine speed on an almost real time basis.
Most methods of producing a digital rotational speed (r.p.m.) signal depend upon generating a pulse train with a frequency proportional to the rotation rate. Although the pulse rate could be at the same rate as the rotation of the measured shaft (i.e., one pulse per revolution) or at some submultiple of the measured shft speed, it is more common to generate a multiplicity of pulses corresponding to each revolution of the measured shaft as, for example, mounting an electromagnetic pickup coil adjacent to a gear on the rotating body, electro-optically sensing the passage of holes or slots in a rotating disc, or the like. Use of a multiplicity of pulses per revolution permits a more accurate digital representation of the instantaneous rotation rate. Since the pulses so generated are actually representative of angular position, conversion to rotation rate requires the introduction of the element of time. This is customarily done either by a frequency measurement, in which the number of pulses representative of the shaft rotation is counted over a fixed (or at least known) time interval, or by a period measurement, in which the number of increments of some short time (i.e., a known high frequency pulse train) is counted between occurences of the rotation-derived pulses or a known multiple of the rotation-derived pulses.
The frequency measurement method is inherently limited at low rotation speed; as the number of rotation-derived pulses in the reference time interval becomes less, the digitized accuracy becomes poorer. The period measurement method is inherently limited at high rotation speeds as the number of time increments during the interval between rotation-derived pulses becomes less, and likewise the digitized accuracy becomes poorer. Period measurements usually suffer from the additional disadvantage that system constraints render it difficult to count each period between successive shaft-related pulses unless special techniques are employed or high-speed memory transfer with simultaneous counter reset is applied. Additional constraints are imposed in a control system by the required control response time (usually short) and, in microprocessor-derived control systems, the requirement that a value for the measured variable (i.e., the shaft speed) that is as current as practical be available to the microprocessor on demand. Further, the cyclic operation of the control microprocessor is completely asynchronous with the shaft rotation.
The U.S. Pat. to Shibata, No. 3,892,952, shows a digital speed detector generating pulses having a frequency proportional to the vehicle speed being measured, a reference pulse generating circuit for generating references pulses having a predetermined frequency, and a timing pulse generator for generating reference timing pulses. In this system the leading edge of a reference timing pulse and a detector pulse representing rotational speed are synchronized with the leading edge of a reference pulse. The trailing edge of the reference timing pulse is effectively extended to the leading edge of the first speed detector output pulse appearing after the end of the duration of the reference timing pulse to determine a counting period. The number of detector output pulses (speed pulses) and the number of reference pulses received during the extended counting period are counted by first and second counters, respectively. The counts of the first and second counters are subjected to a division operation to obtain a displacement speed which, in turn, is subtracted from the previously obtained displacement speed. In this system the counting period is equal to the duration of the reference timing pulse extended by less than one pulse period of the sensor output pulses, which is alleged to provide reduced differentiation error and shorter response time.
One drawback to the system described above is that the time between speed measurement readings is a variable depending upon the speed frequency being measured. In a digital system where the cycle time may be in the neighborhood of 20 msec. this variable will become quite significant at low frequencies. Another disadvantage of this system is that it appears that one speed input pulse is effectively disregarded between speed measurements which tends to further restrict low speed accuracy and the minimum low frequency signal which can be processed.